Liquid dispenser systems, such as liquid soap and sanitizer dispensers, provide a user with a predetermined amount of liquid upon actuation of the dispenser. In addition, it is sometimes desirable to dispense the liquid in the form of foam by, for example, injecting air into the liquid to create a foamy mixture of liquid and air bubbles by use of an air pump or air compressor. Most pumps, whether liquid pumps or foam pumps have a constant volume output and to change the volume requires one to change the pump or “short-stroke” the pump. A pump is short-stroked when the actuator of the dispenser is prevented from moving its full stroke. Problems often occur with pumps when they are short-stroked. If a blocking plate is added to the dispenser actuator so that the actuator does not drive the liquid piston the full length of the pump chambers, many pumps will not prime because an air bubble remains in the liquid piston. Another problem is that air trapped in the liquid dosing chamber results in an inconsistent output.